1. Field of the Invention
The present invention generally relates to an apparatus which provides redundancy of communication routes in a ring network. More specifically, the present invention is directed to such an apparatus capable of realizing high-speed switching of routes, and capable of simply coupling a plurality of ring networks, and also capable of independently performing operations in each of these ring networks so as to realize constructions of more flexible networks, and further, capable of reducing loads given to a CPU.
2. Description of the Related Art
While networks are constructed, there are some possibilities that a plurality of apparatuses are connected to each other in ring structures in order to make communication routes redundant. Networks having ring structures have a feature that necessary amounts of transfer routes and the like may be reduced, as compared with those of networks having mesh structures. On the other hand, since the networks are structured in ring shapes, when broadcast packets flow through the networks having the ring structures, transfer operations of these broadcast packets are repeatedly carried out in an infinite time, so that such a loop is produced in which the broadcast packets are continuously circulated in the same route for a long time. As a result, traffic loads on the networks caused by broadcast streams are increased, or CPU loads caused by unstable conditions of learning are increased, which may give adverse influences to communications. In order to avoid the occurrence of this loop, various methods capable of logically cutting off loop structures have been proposed in networks having ring structures.
In the spanning tree protocol described in IEEE Std 802.1D, 1998 Edition, such a control frame called as “BPDU” is periodically exchanged among apparatuses which construct a network in order to cut off a loop structure. While a plurality of protocol information used to determine operations of the protocol have been superimposed on the BPDU, each of these apparatuses which receive this BPDU changes logical statuses in ports which constitute a loop into blocking statuses based upon the protocol information of the received BPDU, and constructs such a network having a tree structure, so that the loop is logically cut off.
While the redundant protocol described in JP-A-2004-201009 is specifically used in a ring network, one structural apparatus among apparatuses which constitutes the ring network is defined as a monitoring apparatus, whereas other structural apparatuses are defined as relay apparatuses. In the monitoring apparatus, a control frame is periodically transmitted from an one-sided port thereof, and a reception of this transmitted is monitored at an opposite-sided port thereof. While the control frame is being received at the opposite-sided port, the monitoring apparatus transmits another control frame from this opposite-sided port, and sets the opposite-sided port to such a condition that a user frame cannot be relayed. As a consequence, the loop structure is logically cut off. If the control frame cannot be received at the opposite-sided port, then the monitoring apparatus stops the transmission of the control frame from this opposite-sided port, and permits the relay of the user frame. As a result, the communication routes can be made redundant.
Within the above-described conventional techniques, since the spanning tree protocol disclosed in IEEE Std 802.1D 1988 Edition may also be directed to complex network structures, the tree structure must be determined based upon the plurality of information superimposed on BPDU exchanged among the apparatuses, and thus, the protocol operations become complex. As a result, even when a structure of a ring network is made simple, the below-mentioned problems may occur. That is, while the structure of such a simple ring network is changed when a failure occurs and the failure is recovered within this simple ring network, and also while apparatuses are additionally conducted to the simple ring network, there are some possibilities that several tens of seconds are required until communications are stabilized, during which the communications are stopped. Also, since it is practically difficult to realize the complex protocol operations by employing hardware, these complex protocol operations are realized by employing software. As a result, if loads given to a CPU are increased, then there is another problem that an adverse influence may be given to the complex protocol operations.